1. Field of the Invention
This invention relates to a pattern exposure method using a polychromatic light source, and more particularly to a pattern exposure method whereby a predetermined amount of sensitizing energy may be imparted to a photosensitive layer.
2. Description of the Prior Art
The pattern print-out methods heretofore used for the formation of semiconductor integrated circuits or the like are generally grouped into two types, that is, the contact print-out method and the projection print-out method.
The former method has long been known and comprises bringing a pattern and a pellet or like element having a photosensitive layer on which print-out is to be effected into intimate contact with each other, and applying a uniform irradiation to the pattern to thereby print out the image of the pattern on the photosensitive layer. According to this method, the pattern and the photosensitive layer are brought into intimate contact with each other and this eliminates the need to provide a focusing lens between the print-out light source and the pattern, with a result that the print-out light source is free from the problem of chromatic aberration. Thus, a polychromatic light source having two or more wavelength components may be used as the print-out light source and this necessarily leads to a merit that the high brightness of such light source reduces the length of time required for the print-out. On the other hand, however, the high brightness of such light source might result in an adverse effect such as turn-about of the light unless good contact is maintained between the photosensitive layer and the pattern.
In the latter method, the pattern to be printed out is placed in spaced relationship with the photosensitive layer and the pattern image formed by illuminating the pattern with the light source is focused on the photosensitive layer through a focusing lens. According to such method, the pattern need not always be equal in size to the pattern image to be printed out on the photosensitive layer but the pattern image may be reduced in size by means of the focusing lens. Thus, this latter method is effective especially where an extremely miniature pattern is to be printed out. However, the optical means such as focusing lens or the like which is indispensable to this method involves the problem of chromatic aberration and thus, the light source available for this method is restricted to a monochromatic light source or at best to a dichromatic light source if used with a lens for the correction of chromatic aberration. Such light source is necessarily low in brightness, and this in turn leads to an undesirably increased time required to print out a predetermined pattern, thus resulting in a reduced efficiency of operation. Moreover, where the pattern print-out is carried out for a long period of time, fogging of the resultant print occurs because the turnabout of the light is far more serious than in the former method.
Further, the above-described two methods suffer from a common problem which occurs where a substrate such as aluminum, silicon, or like material having a substantial reflecting characteristic, is provided on the back side of the photosensitive layer. If the back side of the photosensitive layer is provided with a reflective surface, the pattern light thrown upon the photosensitive layer is partly absorbed in the photosensitive layer, whereafter the unabsorbed part of the light beam passes through the photosensitive layer and is reflected from the reflective surface to create polymerization in cooperation with the incident light.
Where the light source in use is a monochromatic light source, a standing wave is created within the photosensitive layer and the sensitized amount of the photosensitive layer differs from peak to peak, i.e. from the summit to the valley of the energy distribution of the standing wave. Such phenomenon occurs not only with a monochromatic light source but also with a polychromatic light source. However, in case of a polychromatic light source, which has a high degree of brightness, it is very likely that the photosensitive layer is sufficiently sensitized by the small amount of sensitizing energy present in the valleys of the standing wave and thus, the regions of the photosensitive layer corresponding to the summits of the standing wave are subjected to overexposure. Therefore, in case of a photoresist layer of photosensitive material in an integrated circuit pattern having no half tone, the variation in the energy distribution of the standing wave would result in no irregular exposure if the small sensitizing energy in the valleys of the standing wave is at a sufficient level to completely sensitize the photosensitive layer. However, in case of a monochromatic or dichromatic light source which is not so high in brightness or in case where a polychromatic light source is used with the exposure time reduced, it is impossible to accomplish pattern print-out with the aid of a uniform energy distribution of the standing wave.
A conventional pattern print-out device, especially one which adopts the projection print-out method, is schematically shown in FIG. 1 of the accompanying drawings. This includes a light source 1, and a filter plate 2 having filters 2.sub.1 and 2.sub.2 formed in marginal portions thereof. The filter 2.sub.1 passes therethrough only the light which does not sensitize the photoresist layer of photosensitve material which will be described further, and the filter 2.sub.2 passes therethrough the light which sensitizes the photosensitive layer. The device further includes a reflector plate 3, a collimater lens 4, a pattern 5 to be printed out, a half-mirror 6 for viewfinder, a focusing lens 7, semiconductor wafer 8 comprising a silicon substrate 9 covered with a layer 10 of SiO.sub.2 which in turn is covered with a photoresist layer 11, and a viewfinder's optical system 12-14. Alignment between the pattern 5 and the wafer 8 is effected by means of viewfinder's eyepiece 14, and then the filter 2.sub.2 is disposed in the optical path between the light source 1 and the mirror 3, whereby the image of the pattern 5 is projected through the focusing lens 7 upon the photoresist layer of the wafer 8. The photoresist layer and the SiO.sub.2 layer are substantially equivalent optically and have substantially the same refractive index, so that the print-out light beam impinging on the wafer 8 passed through the photoresist layer 11 and the SiO.sub.2 layer to the silicon substrate 9. Since the silicon has a refractive index of about 30%, the reflected light from the surface of the silicon layer polymerizes in cooperation with the incident light to produce a standing wave as shown in FIG. 3. As a result, within the photoresist layer, the energy distribution of the light beam for sensitizing such layer is varied in the direction of the thickness of the layer 11 by the standing wave.
Where the photoresist layer is formed of a photosensitive resin material having a photo-polymerization characteristic, as is usually the case, the sensitizing energy differs from the summits to the valleys of the standing wave shown in FIG. 3, and this necessarily leads to the difference in the amount of photo-polymerization in the direction of the thickness of the photoresist layer. After the above-described print-out process, when the photoresist layer is developed by a chemical treatment, the portion of the photoresist layer which has not been exposed to the light is removed. However, the amount of photo-polymerization is variable in the interface of the photoresist layer to be removed, as described above, so that the wafer obtained after the etching process will present fine wrinkles in the interface after removal of the photoresist layer, as is indicated by dotted lines in FIG. 4. Thus, even if the resist layer 11 indicated by the dotted lines in FIG. 4 is removed, the SiO.sub.2 layer will also present a rugged contour as a result of the non-uniform etching effect. Thus, precise etching of the pattern image cannot be achieved.
Especially, where the pattern to be printed out is extremely miniature, the wrinkles so formed would prevent precise representation of the pattern to be printed out and thus, highly accurate print-out would be impossible.
In addition to the above-described contact print-out method and projection print-out method, there is known the off-contact print-out system which is an intermediate between the two methods. This system is such that print-out is effected with the pattern to be printed out being spaced apart slightly, say, several tens of microns, from the photosensitive material or, during the manufacture of integrated circuits, the photoresist layer applied to the semiconductor chip. According to this system, the contrast of the pattern image projected upon the photosensitive material is lower than in the contact printout system, as is well known. This is attributable to the fact that a monochromatic light source comprising a specific wavelength is most preferred in this system because the use of other various light sources for print-out suffers from the problem of diffraction. Therefore, if print-out is to be effected on the surface of a print-out chip having a reflective surface provided by the lower surface of the photosensitive layer as described above, the amount of exposure will differ from the valleys of the resultant standing wave having a low intensity of sensitizing energy to the summits of the standing wave having a high intensity of sensitizing energy. Thus, if an energy sufficient to sensitize the valleys of the standing wave is imparted, the regions subjected to a sensitizing energy corresponding to the summits of the standing wave will be overexposed so that precise representation of a miniature pattern will be impossible, as will be appreciated from the foregoing description. Especially, where a photosensitive resin material having a photo-polymerization characteristic is used as the photoresist, the pattern width subjected to the over-exposure will be greater than the pattern to be printed out. Thus, the off-contact system suffers from a disadvantage quite similar to that peculiar to the projection print-out method.